1. Field of the Invention
The invention has a two-fold purpose,
i) a method and apparatus for rapidly and automatically measuring dimensions of fasteners by non-contact methods, and PA1 ii) an instrument which is compact, rugged and reliable enough to be used under a variety of adverse conditions.
The invention employs imaging to obtain a two dimensional pixel image silhouette subject to analysis whereby precise measurements are performed. The instrument's hardware is composed of minimal components to minimize sources of errors and is compact to be hand held and rugged enough to be used on a factory floor without any concern for instrument malfunction. The invention is a non-contact gaging method which automatically and rapidly gauges the dimensions of precision objects.
2. Prior Art
A higher quality product (fastener) leads to longer life of the end product where they are used. Most of the instruments available to the market are contact based and tend to be sluggish in their operation and require experience for its proper use on the part of the operator. The non-contact based instruments available to the industry are still not optimized for their speed and are quite expensive to be readily accessible to a wider market. On the other hand optical based instruments available to the industry at present do not lack speed but lack the tolerances as required by the industry. In numerous applications, particular dimensions of the fasteners are critical as to how precise, that is, how smoothly the fasteners engage mating fasteners. The author feels that there is a need for an instrument which possesses the following properties--high speed gaging, accuracy, minimal or no alignment requirements, and priced so that the instrument is readily affordable.
The following parameters need to be determined whereby the quality of fasteners can be assessed; functional pitch diameter, pitch diameter, minor and major diameter, drunken helix, shank diameter, perpendicularity, eccentricity, thread pitch, pitch diameter, flank angle, length of thread engagement. This instrument allows for the possibility of measurements to be accomplished as quickly as the parts are produced, whereby 100% quality assurance becomes a possibility and hence, a reality. Presently, tradeoffs are made between quality assurance measurement and speed, for example by taking measurements on a small percentage of the fasteners from the batch.
Prior art takes at least minutes to accurately gauge relevant fastener's dimensions. Because of this, it is not possible to timely gauge reasonable numbers of fasteners in a batch to determine if the batch falls within a specific tolerance or as a matter of fact, conduct a Statistical Process Control in a manner such that a bottleneck is not formed. It is not uncommon that only a very small percentage of the fasteners in the batch be gauged under the Statistical Process Control.
Under the Fastener Quality Act it will be required to sell fasteners with certification of qualified tests conducted on the batch of fasteners with the laboratory conducting such measurements certified by the National Standards Laboratory such as the NIST. The number of fasteners inspected under the SPC is as low as one hundredth of a percent. If the selected group of fasteners passes, the entire batch is accepted. What would be desirable is to provide the industry with a low cost high speed rugged instrument which would allow for higher sampling rate, further improving the standard of fasteners available in the marketplace thereby enhancing the quality of the end product. Even more desirable would be to provide a specific market a 100% measurements for all the fasteners provided. This industry can be such as NASA's space station or high performance aircraft, whose total cost to procure when compared with the cost to conduct 100% inspection is an insignificant fraction. A device comprising independent hardware component variables to conduct certain measurements of an object is susceptible to error by the very process of conducting such measurements. It is desirable to minimize the hardware components involved to conduct such measurements so as to reduce a chance of such problems or failure from occurring in the first place. A scanning arrangement is inherently serial, and as a result, the scanner can sense light intensity from the beam at only unit area at a given time. Timing and precise repetitive motion considerations affect the accuracy of the measurement. Moreover, scanning arrangements are prone to further error if one attempts to apply them to moving parts.
Most of the instruments available in the market employ a lens to expand the point source laser light to encompass an object to be examined, the object is backlighted by use of a light source. The object partly occludes the beam and provides an image. Lenses concentrate the beam at a point or scanning tube arrangement, such that the luminance level is sampled repetitively in conjunction with a threshlod comparator and timing means to locate the edge of the object from a change in the intensity of the luminance signal. The comparator data, in connection with timing data, can be converted into a dimensional measurement. U.S. Pat. No. 3,941,484--Dreyfus, describes the device which employs such method.
U.S. Pat. Nos. 4,576,482, and 5,114,230, both to Pryor, disclose methods in which a collimated laser light is used as a back-light to collect luminance information by finding the transition in luminance across a single edge of an object. The light is partly occluded by the object and is applied to a linear photodiode array having an extension perpendicular to the edge. The object is rotated along its axis to inspect across the diameter to determine major diameter, and to check the quality of fastener threads. Reflected light, because of the scattering from a surface, produces a low signal to noise ratio resulting in low accuracy measurements when used to gauge fastener dimensions. The accuracy of the measurements performed with such methods is limited by several factors, including the fidelity of the input image. Signals are analyzed by this technique, indirectely to extract the relevant information pertaining to the object's dimensions.
Over the last few years considerable improvement has been made in image enhancement technology whereby quality of the captured image is improved considerably. Aberrations, defocused images can be corrected for with the present day image software technology. Images can be improved by edge enhancements, deconvolution, sobels, gradient technique and others. What a software based enhancement is capable of conducting can be best described by the images of the distant galaxy as captured by the Hubble telescope. Remarkable improvement in the quality of the images are evident after the images are processed. So, to the author of the present patent application, to gauge a fastener or part utilizing the image enhancement technology in conjunction with the hardware offers a distinct advantage and a unique capability which is not available to the industry. This instrument, primarily because of afew components, offers a very rugged construction.
Thread gaging is a demanding imaging problem that has not been addressed properly and to our knowledge, not been solved. The three dimensional object's image is captured onto a two-dimensional array. When the image is gauged as is, the measurements do not represent true dimensions of the threaded fasteners.
The primary object of the invention is to calculate and determine from the two dimensional image a three dimensional gauge fastener's dimension and gauge the object's dimensions at high speed correctly and accurately. The helical angle on the threaded fasteners induces error onto the measurements of the pitch diameter of the fasteners and hence to its functional diameter. In this invention, we calculate from the captured image the helical angle and thereby calculate the error introduced on the pitch diameter and correctly estimate the pitch diameter. The invention avoids tilting the fastener to measure the correct pitch diameter. The invention contains no moving parts to accurately gauge fasteners. The errors introduced by capturing a three dimensional object's image onto a two dimensional array are corrected by the software means in this imvention. In essence a two dimensional image is transformed via software means to a three dimensional image in a computer memory and, with this information, accurate dimensions are displayed.
The invention not only eliminates a need for a focusing lens, but also does away with the spatial filter, and the like. Emphasis is placed on the software to gauge accurately as long as the conditions under which calibration is conducted is unaltered during the measurement period. Previously, it was necessary to accurately position the focusing lens in space a certain distance from the object being gauged, in order to produce a focused image/shadow. In the invention, minimizing the distance between the object, and the location of the array is important to gauge. This technique is without a need for a lens and spatial filter, and the accompanying problem of correctly positioning the lens with respect to the object and otherwise focusing the image.
U.S. Pat. No. 4,644,394--to Reeves, discusses examining external threads, on pipe, and/or fasteners. The patent employs a collimated laser beam to back light the threads along one side of the work piece--in his example case, a pipe. A luminance transition is hence employed, and the incident radiation impinging on the work piece and partly transmitting on the work piece is thus detected at a tangent of the workpiece surface. The information obtained as such is an image of the pipe threads is not of only one side of the pipe but is also of only one particular angular point on one side of the pipe. Data as such is collected and processed and enables certain thread measurements in that particular area only. However, in order to make other related thread measurements at other angular points around the pipe, it is necessary to move the part of the instrument precisely to gauge properly while maintaining the alignment. These drive requirements introduce time constraints and measurement inaccuracies. As can be deduced by those familiar with the art of gaging, it is necessary to rely on the accuracy of the rotational drive means and the ability to accurately position the part at different angular positions around the rotation axis. This process not only reduces the time to gauge, but also increases the probability of error due to the moving parts associated during the gaging process.
In the following two paragraphs we discuss the patents issued to the Boeing Air Corporation. U.S. Pat. No. 4,823,396 granted to Thompson discloses a method in which a fastener's dimensions are measured by the imaging technique. The workpiece in this case is a rivet back-lit by an array of light emitting diodes. A video image of the fastener is created which is digitized and processed and analyzed. Various dimensional measurements including the head and shank of the fasteners are conducted. The accuracy of the measurements performed is dependent on the distance between the camera lens and the fasteners. Slight variations in focus and the distance translate to variations in the apparent size of features in the image on the camera detector array, and consequently measurement errors. Such uncertainties are significant when fasteners must be measured to within tight tolerances, for example, for use in the aerospace industry.
The other two patents assigned to the Boeing Air Corporation and issued to Woods, that is U.S. Pat. Nos. 4,828,159 and 5,150,623 disclose imaging methods to gauge fastener dimensions. The first patent discloses methods which use two cameras, imaging the fastener from two orthogonal directions and mathematically calculating the position of the fastener by means of the focussing distances. Hence determination of precise fastener position is dependent upon precise camera focussing. An out of focus image leads to an error in calculation of the fastener's position and translates into error in measuring the fastener's dimensions. It is desirable to have an instrument which does not have the above mentioned drawbacks.
Furthermore the ideal circumstance, where this technology is available on every engineer's desk is if the instrument can be constructed in a manner that is not only rugged but is also compact enough to be conveniently transported without the problems of misalignment. Such problems are compounded if the spatial filter is involved to remove the random noises and collimating optics for the beam. The technology has progressed rapidly; stabilizing the source's noise such that accurate measurements can be performed without any intervening optics or optical components is available to the industry, but is cost prohibitive to be a part of the end product. Whenever optical noises are present, fluctuations due to the source offer minimal or no reduction in the accuracy required for gaging a fastener, rivets or the like, as required by the stringent MIL Standards. This is the strength of the present patent and emphasizes this point as one of the primary motivations behind the invention.
The basic idea behind the present invention disclosed in this patent is that the instrument collects and analyzes a two dimensional image of an object by employing digital imaging means and conducts dimensional measurements. An image of an object and hence its profile, is captured on a two dimensional detector array. The image is digitized by means of a frame grabber and analyzed by image enhancement techniques intended to produce better edge contrast. Even further precision is achieved by using subpixel algorithms. A hardware means such as a digital computer or digital signal processor processes the digital information to analyze the image to locate edges and other relevant image parameters. At all times, the high precision scale etched on the faceplate of the fiberoptic column is gauged for its dimensions and compared with the stored information. The stored information is obtained via standard contact based gaging methods, methods which are well accepted by industry. Any difference in the comparison is compensated for, which translates to beam divergence or convergence etc. This process eliminates the need for maintaining high stability and alignment of the optics at all times. The process in essence ruggedizes the instrument. Various dimensions of the objects are determined by this method from the pixel position displacements between transition positions and luminance. In light of the drive by the microprocessor industry for faster and cheaper processors, with a proper setup, measurements can be made quickly and accurately even of objects moving rapidly on an assembly line conveyor.
In the following paragraphs, we discuss the patents which appear to cover certain aspects of our invention. It should be pointed out that gaging is not the motivation of these prior art patents and hence they do not fall into the envelope of our claims. Nevertheless, these patents will be discussed here to point out distinct differences between our invention and these prior art patents.
U.S. Pat. No. 4,777,360 issued to Don C. Carner, Jr. titled "Umbra/Penumbra Detector" uses the camera detector array without the use of a focusing lens. The invention is a lensless direct-contact or "shadow-casting" imaging system which is used in conjunction with any image sensing substrate. The device monitors the characteristics of an image cast upon a radiation sensitive substrate. The image can be cast by the shadow or a mask and at least one source of radiation disposed above the object or mask so that the image can be cast on the substrate and analyzed. The patentee points out that the image provides the means by which two, three, and four dimensional data representations can be made of any environment where radiation is present. This is an ideal instrument for gaging as long as the object to be gauged is a two dimensional object. For example, if a fastener is gauged with this instrument, due to the three dimensional nature of the fastener, and in particular to its helical angle, if one takes it's two dimensional image and gauges using this image, it will calculate the dimensions incorrectly. We have determined that, for the fasteners manufactured within the tolerances, the errors associated with the pitch diameter by this method fall, incorrectly, well outside the actual dimensions and tolerance requirement as demanded by the industry. We compared the measurements as obtained by this instrument against the measurements as obtained by other established methods of gaging. This method was used to extensively test using different numbers and types of fasteners; and despite the fact for fasteners whose measurements fell within the tolerance requirement with other established methods, the measured values came out quite wrong with this technique. Extensive software need to be written to correct for these discrepancies. For these reasons, our patent differs significantly from the patents of the others. Not mentioned is: one needs to employ subpixel algorithms to gauge the fasteners to the accuracy as demanded by the industry. If one gauges without the use of the subpixel routine, the errors associated with these measurements exceeds by far the required accuracy for the measurements and hence the instrument is of no relevance to the industry.
The cost associated with larger size CCD/CID analysis is prohibitively expensive to employ other means for gauging. Fiber optic coupling is used to reduce the size of a fastener to be gauged, such that when the image is captured, the full diameter of the fastener image is captured. Moreover, the fiber optic coupler allows moving the optic axis of the camera's array significantly away from its original position for any practical use.
U.S. Pat. No. 5,327,217 issued to Kanai et.al. titled "Apparatus For Measuring Particulate Size Without Contacting the Particulate" uses the apparatus for measuring the size of a particulate, without contacting the particulate. Stripped gaps of interference fringes are measured behind the particulate when a monochromatic beam is radiated on the particulate. In this patent the particulate is placed at a distance from the detector array and the expression is derived to calculate its dimensions from its interference fringes. This is an innovative as well as clever scheme, but differs in its entirety from the present invention. Kanai et.al. enhances the particulate fringes and the present patent is attempting to minimize this effect. Moreover, the expression derived in their patent is not relevant for our purposes because the distance from the camera array to the object which is to be measured, is finite. In contrast, we keep the distance between the object (fastener) and the camera array to minimum and in this domain a mathematical expression is inapplicable. Moreover, if one uses the technique of Kanai et.al. for gaging fasteners the diffraction patterns interfere with the adjacent threads creating a very complicated mesh incapable of allowing the operator to generate ANY information from the silhouette.